1. Field of the Invention
This invention relates to toner cartridges for electronic printers and more particularly to metering or “doctor” blades for regulating the feed of toner to image elements from a feed or developer roller.
2. Background Information
Electronic or “laser” printers use a focused light beam to expose discrete portions of an image transfer drum so that these portions attract printing toner. Toner is a mixture of pigment (typically carbon black or a non-black color component) and a plastic component, which is typically polystyrene or polyester. The toner becomes electric statically attracted to exposed portions of the image transferred drum. As a transfer medium such as paper is passed over the rotating image transferred drum, some of the toner is laid onto the medium. Subsequently, the medium passes through a heated fuser so that the toner's plastic component is melted into permanent engagement with the underlying medium.
The vast majority of desktop laser printers currently available utilize replaceable toner cartridges that incorporate an image transfer drum, a toner tank and a metering system and a drive mechanism for the drum and metering system. An exemplary toner cartridge toner tank assembly 100 according to the prior art is shown in FIG. 1. As described above, the toner cartridge of this example can be adapted to install in the E320/E322™ series printer, available from Lexmark®, and operating in a compatible print engine. Note that, while a particular Lexmark cartridge is shown herein, this description is meant to apply to a wide variety of cartridges by this manufacturer and others, all of which employ similar principles for feeding and metering toner. The cartridge includes a toner tank housing 102 that defines generally a cylindrical shape. Within the tank, along the bottom is located a toner supply 104 consisting of an electrostatically attracted particulate compound. In this example, the toner is a “one-part” toner having a color (carbon black, for example) infused with a melting substance, such as polystyrene plastic. Toner is deposited by a feed or metering section 106, having a foam-surfaced supply roller 110, elastomeric (urethane) feed or “developer” roller 112 and metering blade 114, onto an electrostatically charged image transfer drum 116. The drum 116 is charged in a particular pattern that corresponds to the pattern laid down by a coherent light source (e.g. a laser) L. The patterned toner is transferred from the drum 116 to a print media (paper for example) as it passes by the drum in synchronization with its rotation. The drum and other components of the cartridge are generally part of an overall outer cartridge housing that includes the tank assembly 100 and feed section 106 as well as certain sensors and control electronics.
The toner is continuously agitated and urged from the “sump” of the tank bottom to the feed section 106 by rotation (curved arrow 118) of an agitator paddle 120. The paddle 120 is formed as a framework with a leading edge supported on a series of ribs that are, in turn, connected to a central axle 124. The central axle 124 is rotationally supported at the center of the tank cylinder. During paddle rotation, the paddle sweeps through an arc that passes just above the inner surface of the tank, while the ribs cut through the toner, enabling the toner to pass through interstices defined therebetween. In this manner, the leading edge serves to break up and drive the toner upwardly into the feed section 106 though slots 130 in the tank.
The feed section rollers and agitator paddle are driven by a printer engine drive motor (not shown) that engages an external gear train (not shown). This gear train interconnects and drives the paddle 120, the foam-covered supply or “adder” roller 110, urethane-surfaced metering or developer roller 112 and image drum 116 in rotational synchronization about respective axes of rotation 124, 140, 142 and 146.
The developer roller 112 is particularly adapted to carefully meter the amount of toner delivered to the exposed (attractive) parts of the image drum 116. To ensure that excess particles do not adhere to the developer roller 112 before it releases the toner at the release point 152 (a point of closest proximity between developer roller 112 and image drum 116), the metering blade 114 is carefully positioned above the developer roller's (112) surface. The blade is spring-loaded to exert a downward force (arrow DF) of approximately 16 ounces (in this example) against the elastomeric surface of the developer roller. Controlling spring pressure on the blade is an important factor in properly metering toner using this blade style. A lighter spring allows more toner to pass through, and vice versa. Similarly, the location of upstream corner 170 of the blade's working face and the surface of the roller 112 is set precisely and aligned with respect to the axis 142 (and associated radial centerline 172) to achieve an even metering of toner across the axial length of the roller 112. In operation, the corner 170 serves to block the majority of toner particles picked up by attraction to the roller surface. A relatively thin film of particles manages to pass between the impingement point between the blade corner 170 and the roller surface. This film is selectively released to the image drum for subsequent transfer to the printable substrate. Any remaining excess toner after release to the image drum passes by semi-rigid plastic (typically Mylar) strip 160 that acts as a barrier between the bottom of the toner sump and the developer roller 112. Downstream of the barrier strip 160, new toner from the sump is attracted onto the developer roller 112 to combine with the preexisting toner that remains after release to the drum. This new film presents itself to the blade 114 for metering. Note that a small amount of toner may also fall from the roller 116 and/or drum 116 outside of the feed section 106. This errant toner is collected in a waste area (not shown) that is beneath the feed section 106 in an adjacent section of the outer tank housing (also not shown).
To provide a more-even release of toner from the developer roller 112 to the image drum 116 (and thereby reduce toner waste and increase overall print quality), it is desirable to improve the metering of toner before it is presented to the image drum. The metering blade 114 is, essentially, the last opportunity to properly regulate toner supply before release to the image drum. It is, thus, desirable to improve metering blade performance to the greatest extend practicable. The uneven release of toner across the image drum leads to irregular print quality. However, it is not uncommon for a newly manufactured toner cartridge to exhibit significant variation in toner metering between each of opposing ends of the developer roller 112. Often the variation results from slight misalignment of the roller's axis/centerline with respect to the working face of the blade 114 and its upstream corner. Even a one-thousandth-of-an-inch misalignment may significantly affect print quality. In addition, irregular toner metering across the roller can result from electrostatic differences across the roller or blade's surface. Both the roller and blade are charged by AC and DC current to cause toner to be attracted to the roller. The urethane roller may exhibit dielectrically created differential across it length. This electrostatic differential may cause uneven metering even where the blade and roller are perfectly aligned. Finally, even where a cartridge is finely tuned to provide metering evenly across its length, the metering may become variable simply by employing a different type or batch of toner with slightly different granularity and/or electrostatic properties. A technique for quickly, predictable and efficiently dealing with all these causes of metering variability is highly desirable.